Fix the core issue reported in #30397.

src/EditorFeatures/CSharpTest/RemoveUnusedMembers/RemoveUnusedMembersTests.cs

@@ -6,6 +6,7 @@
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.Diagnostics;
 using Microsoft.CodeAnalysis.Test.Utilities;
+using Roslyn.Test.Utilities;
 using Xunit;
 using static Roslyn.Test.Utilities.TestHelpers;
 
@@ -1076,6 +1077,7 @@ int Goo
         }
 
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnusedMembers)]
+        [WorkItem(30397, "https://github.com/dotnet/roslyn/issues/30397")]
         public async Task FieldIsIncrementedAndValueUsed()
         {
             await TestDiagnosticMissingAsync(
@@ -1087,6 +1089,7 @@ public async Task FieldIsIncrementedAndValueUsed()
         }
 
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnusedMembers)]
+        [WorkItem(30397, "https://github.com/dotnet/roslyn/issues/30397")]
         public async Task FieldIsIncrementedAndValueUsed_02()
         {
             await TestDiagnosticMissingAsync(

src/Features/Core/Portable/RemoveUnusedMembers/AbstractRemoveUnusedMembersDiagnosticAnalyzer.cs

@@ -194,7 +194,7 @@ private void AnalyzeMemberReferenceOperation(OperationAnalysisContext operationC
                             compoundAssignment.Target == memberReference ||
                             memberReference.Parent is IIncrementOrDecrementOperation);
 
-                        // Compound assignment or increment whose value is being dropped (parent has null type)
+                        // Compound assignment or increment whose value is being dropped (parent is an expression statement)
                         // is treated as a Write as the value was never actually 'read' in a way that is observable.
                         //
                         // Consider the following example:
@@ -209,7 +209,7 @@ private void AnalyzeMemberReferenceOperation(OperationAnalysisContext operationC
                         // while the increment operation '_f2++' is child of a return statement, which uses the result of the increment.
                         // For the above test, '_f1' can be safely removed without affecting the semantics of the program, while '_f2' cannot be removed.
 
-                        if (memberReference.Parent.Parent?.Type == null)
+                        if (memberReference.Parent.Parent is IExpressionStatementOperation)
                         {
                             valueUsageInfo = ValueUsageInfo.Write;
                         }